Soft lithography is a versatile patterning technique for use in microfabrication to produce microstructures. This technique uses a patterned elastomer to transfer a pattern from a master to a substrate. The patterned elastomer may be used, for example, as a stamp to transfer a substance, as a mold to be filled by a substance, or as a mask to provide for selective deposition onto a substrate and/or selective removal of material from the substrate. See, for example, Xia, Y. and Whitesides, G. M. Annu. Rev. Mater. Sci. (1998) 28:153-184.
In contrast, conventional photolithography utilizes rigid photomasks to pattern photoresist layers, and the patterned photoresist then protects the material beneath the pattern during subsequent etching and deposition steps. Soft lithography provides a number of advantages over conventional photolithography. Soft lithography can yield three-dimensional structures and non-planar structures in a single deposition step, rather than requiring the stepwise assembly of individual layers. Due to the mechanical flexiblility of the elastomer, non-planar substrates can be patterned. The various soft lithographic techniques can also be used with a more diverse range of materials than are available with photolithography, and the materials and techniques used in soft lithography are typically much lower in cost. Because of these advantages, soft lithography has proven useful for applications including integrated optics, microelectromechanical systems (MEMS), microfluidics, and patterning of biological material such as proteins, nucleic acids and cells.
In one example, a patterned elastomeric stamp can be contacted with a substrate to form channels which can exhibit a pattern in two or three dimensions. The channels can then be filled with a liquid precursor for a solid substance, such as a polymer or a ceramic. The channels can also be used to mix different fluid substances, thus functioning as a microreactor or a microanalytical tool. The formation of solid patterned structures with this technique is referred to as Micromolding In Capillaries, or “MIMIC.” Drawbacks to this technique include the need for the pattern to be continuous to allow the entire pattern to be filled. Also, the channels must be large enough to accommodate the viscosity of the liquid used to fill the pattern, which can limit the resolution that can be obtained.
In another example, a patterned elastomeric stamp can be coated with a substance and then contacted with a substrate. Removal of the stamp results in a deposition of the substance onto the substrate in the pattern of the stamp. The substance which is transferred thus functions as an ink which is printed onto the substrate. This technique, referred to as microcontact printing or “μCP,” can be used to form discontinuous patterns, and can form patterns with higher resolution than MIMIC. Applications of microcontact printing typically involve additive lithography, which is the selective deposition of another substance on either the patterned ink or on the exposed substrate. Drawbacks to this technique include the limited range of materials which can be used as the substrate and as the ink.
In yet another example, a patterned elastomeric membrane can be applied to a substrate. This membrane can then function as a mask for selective removal of the exposed substrate (subtractive lithography), or for additive lithography. Depending on the materials used for the membrane and the substrate, reversible bonding between the two can be used to stabilize the membrane during the desired microfabrication and to remove the membrane once it has served its intended purpose. Drawbacks to this technique include the extreme difficulty in applying, removing, and manipulating the thin elastomeric membrane. Also, the membrane must be continuous and cannot be used for imaging discrete forms and patterns.
It is thus desirable to provide an improved soft lithographic technique that can be used to form patterns that are continuous or discrete, two dimensional or three dimensional, on planar and non-planar substrates, and that may be in the form of channels or masks for additive and subtractive lithography. It is also desirable that these patterns can be formed on and with a wide range of substances, without the need for delicate handling of the materials involved.